If you have an active subscription, which is what was created when you activated on the first computer, you need to use the same LiveID on the subsequent activations. You can't use the same key twice. If you are arriving at a screen to purchase or provide a token, you are likely not using the same LiveID as the subscription ID.
If you don't recall the LiveID you used for your subscription, you will need to contact support who can provide that information to you using your Key and/or contact information.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
Alternatively, sign in at http://billing.microsoft.com with email addresses that you have set up as LiveIDs and one will show your subscription listed - that would the LiveID to use to activate the reinstall.
If you already entered an incorrect LiveID in an attempt to activate, unfortunately, you've encountered a bug in 2.0 that will be fixed in the future. Once you enter a LiveID in OneCare for use in the trial version to enable multi-PC functionality, you cannot change that when you wish to activate a subscription. Your only solution at this point is to uninstall OneCare and then reinstall which will allow you to enter the correct LiveID.
-steve
